Graphene, as one of the allotropic forms of carbon, is regarded as the final element of the series of polycyclic aromatic hydrocarbons. Graphene is composed of a monolayer of carbon atoms arranged in hexagonal rings which are linked to one another. Since the material is just one-atom thick, the form is referred to as a two-dimensional structure of carbon atoms packed into a hexagonal lattice. The length of the carbon-carbon bonds is ca. 1.42 Å. Carbon atoms in graphene form a flat, practically two-dimensional lattice with hexagonal cells, resembling a honeycomb pattern.
There are known graphene production methods which make use of the liquid state for graphene formation but are not based on the variable decreasing solubility of carbon in the liquid state. The method outlined in the patent application no. US 2012/0082737 relates to the formation of a graphene layer using liquid gallium. According to the method, an amorphous carbon film produced by vapour-depositing on an organic film is then transferred to a substrate of liquid gallium (or, alternatively, indium, tin or antimony), forming a graphene layer as a result of the graphitization reaction arising at the contact interface between the solid phase and the liquid phase. The other known method, presented in the patent description no. US2010/0055464, consists in that an eutectic mixture containing pure graphite and a solvent selected from Ni, Cr, Mn, Fe, Co, Ta, Pd, Pt, La and Ce releases, in the process of crystallization, graphite which is then separated into individual graphene layers. To this aim, a graphite disc is placed on a plate made, for example, of pure nickel, heated to 1,500° C. for 30-60 min, vacuum annealed at a pressure of 1.33−5 mbar and then cooled down slowly.
Known are also methods based on the epitaxial growth of graphene from the gaseous phase on substrates composed of metals, alloys and intermetallic phases with mono- or polycrystalline structures. The transport of carbon into the graph layer often makes use of the variable solubility of carbon, decreasing along with temperature, in the solid state forming matrix.
Known from the patent description no. US 2011/0108609 is a method of fabricating graphene in which a layer including nickel (Ni) or a nickel-based alloy containing at least one metal from the group comprising copper (Cu), iron (Fe), gold (Au) and platinum (Pt) is saturated with carbon. Nickel-based alloys contain between 5 and 30 atomic % of Ni. The role of alloy additives is to restrict the solubility of carbon in Ni. The catalyst layer is applied in the process of evaporation to a silicon substrate coated with silica (SiO2/Si). Stacked catalyst layers can be obtained by sputtering, evaporation or the CVD method, or by alternately stacking thin metal films atop one another, followed by annealing. The thickness of the layers varies from ca. 10 to ca. 1,000 nm. In order to produce a graphene layer, hydrocarbons are supplied into the processing chamber containing a previously applied catalyst layer. Argon may also be supplied with the hydrocarbons as a carrier gas. The ratio of the mixture containing a carbon-carrying gas (usually acetylene) and argon is 1:40. During the formation of the graphene layer the temperature of the substrate can be in the range of 650° C.÷750° C. The graphene layer is produced as a result of variable carbon solubility in the catalyst material of the substrate.
Known from the patent description no. US 2009/0155561 is a method which comprises placing a carbon material directly on a catalyst layer which, as a result of heat treatment, undergoes thermal decomposition thus becoming a source of carbon atoms for infiltrating the catalyst layer. The carbonaceous material covering the catalyst layer can be a polymer applied to the surface using a variety of coating or immersion methods or other techniques in order to create a uniform layer. The thickness of the graphene sheet produced in this manner can be controlled by the molecular weight and the quantity of applied polymer. The next stage involves heat treatment in an inert or reducing atmosphere in order to achieve thermal decomposition of the polymer. As an alternative to placing a carbonaceous material on the surface of the catalyst layer, the catalyst layer can be contacted directly with a gaseous carbon source which, according to the patent's claim, can include a compound from the group comprising carbon monoxide, ethane, ethylene, ethanol, acetylene, propane, propylene, butane, butadiene, pentane, pentene, cyclopentadiene, hexane, cyclohexane, benzene, toluene and their combinations. Carbon atoms infiltrate the catalyst layer until the solubility limit is achieved, which is when graphene nucleation occurs and graphene grows forming individual graphene sheets. A significant element of the processes is the cooling method. Catalyst layers can have a single-crystal structure and comprise a metal selected from the group consisting of Ni, Co, Fe, Pt, Au, Al, Cr, Cu, Mg, Mn, Mo, Rh, Si, Ta, Ti, W, U, V, Zr or alloys containing at least one of the foregoing metals. Since the catalyst layer is monocrystalline, it resolves the problem of defects which occurs in polycrystalline catalysts. The catalyst layer can be used on its own or, as in other methods, disposed on a silicon substrate. The method is suitable for producing between 1 and ca. 300 layers, each single layer measuring between ca. 1 mm and ca. 1,000 mm.
Yet another method disclosed in the patent application no. US 2010/0255984 involves the process of carbonization of ruthenium (Ru) as a mono- or polycrystalline substrate deposited epitaxially with a plane (0001) having a hexagonal crystalline structure matched to that of the emerging graphene. According to the method, the substrate is heated to a temperature of ca. 0.5 Ttop., under reduced pressure, maintained at that temperature for several up to several dozen seconds, and then annealed in an atmosphere of ethylene at 10−5 mbar, following which Ru is cooled down to a temperature of 0.3÷0.4 Ttop. at a rate of 20° C./min or lower.
The patent description no. US 2011/0033688 discloses the process of graphene growth from a gaseous phase containing acetylene or methane with argon (as a carrier gas). Similarly to the methods reported above, the catalyst is a thin layer of metal, suitable metals including nickel, cobalt, iron, copper or metal alloys (e.g. iron-nickel, nickel-chromium) applied to the substrate. The catalyst layer has an oriented polycrystalline structure with large grains, ca. 10 μm in size. The process involves the stage of heating the substrate to 800÷900° C. in a carbon-rich atmosphere at a pressure of 5÷150 mbar, followed by sudden cooling. Cooling is performed with an inert gas, to the temperature level of 700° C. or lower.
Known from the patent description no, EP 2 392 547 is a method based on graphene growth from the gaseous phase on a SiC (silicon carbide) substrate, characterized in that the process of silicon sublimation from the substrate is controlled with the flow of an inert gas—argon (or a gas other than inert) through the epitaxial reactor. The flow rate of the inert gas ranges from 6 l/min (or less) to 70 l/min at a pressure of 10−4 mbar. The substrate becomes heated to over 1,100° C. during the process. The epitaxy is preceded by a stage of etching in the atmosphere of gas containing hydrogen. It may also additionally contain propane, silane or their mixtures, or other hydrocarbons. The process is conducted at various temperatures within the 1,400° C.÷2,000° C. range and at a pressure of 10÷1,000 mbar.
The patent description no. US 2011/0206934 presents a method for producing an appropriate multilayered structure, with one of the layers constituting a source of carbon atoms. The substrate is composed of silicon coated with silica, to which amorphous carbon is applied by ion sputtering, and then hydrogenated. Alternatively, the layer comprises a metal-carbon alloy containing at least 50 atomic percent of carbon, and has a thickness in the range from 0.5 nm to 50 nm. As the next stage, a second layer composed of a metal (Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Ru or their alloys), between 10 and 1,000 nm thick, is applied to the first layer, e.g. using the PVD method. The multilayered structure is then annealed at the temperature range of 550° C.÷1,400° C. in the atmosphere of Ar/H2 and N2/H2, N2, Ar, He or in a vacuum. In this way, carbon infiltrates the metal layer, leading to the deposition of graphene.
All the currently known methods of graphene production utilize substrates, catalyst layers (graphene forming matrices) in the solid state.